The present invention is directed to cathode ray tube display screens, and more particularly to an overlay (such as an optical filter) for CRT display face.
CRT display screens commonly comprise an optical filter overlay panel mounted in laminal relation to the glass display face of the CRT. The purpose of the filter overlay panel is to reduce glare and to enhance contrast (in addition, the filter can be colored to adjust the phosphor colors of the CRT to achieve a desired color display). Typically, the overlay panel is formed of semi-rigid plastic with a glare-reducing matt finish, corresponding in contour to the CRT display face.
Two principal techniques are used to secure the filter overlay panel to the CRT display face. In one technique, the filter panel is bonded directly to the display face using a clear bonding agent, forming a laminate display screen. In the other technique, the filter panel is secured at its perimeter to the CRT such that an air gap remains between the filter panel and the CRT display face; the air gap is required to avoid optical distortion due to intermittent point contact between the filter panel and the display face.
Both of these mounting techniques provide CRT display screens (display face plus mounted filter panel) having a number of disadvantages. In the bonding procedure, care must be taken to insure uniform spacing between the filter panel and the display face to prevent differential thermal expansion and contraction resulting in separation (a number of techniques have been devised to insure such a uniform separation, for example, U.S. Pat. No. 4,204,231 to Permenter). Also, despite the use of a clear bonding agent, undesireable attenuation can occur, thereby reducing screen brightness. Indeed, because the adhesive must bond both to the glass display face and the plastic overlay panel, the choice of a proper adhesive makes it difficult to maximize adhesive transparency, further degrading optical transmission. Further, with respect to the non-bonding technique, the bonding procedure introduces significant additional manufacturing costs. Regarding the non-bonding technique, to insure that a gap is maintained across the entire surface of the display face, manufacturing tolerances require that the gap at the edges of the filter panel where it is mounted to the CRT display face be on the order of 0.031-0.062 inches (typically, this gap is maintained using double sided adhesive tape of a corresponding thickness); disadvantageously, a gap of this magnitude significantly decreases screen contrast and display clarity. Moreover, later CRT assembly procedures may adversely affect the gap spacing between the overlay panel and the display face. Thus, while this mounting technique for a filter panel involves significantly reduced manufacturing costs by avoiding the bonding procedure, in many applications the degredation in display clarity makes its use undesirable.